Convert CONFIG_USB_GADGET_AT91 to Kconfig

This converts the following to Kconfig:
   CONFIG_USB_GADGET_AT91

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/taurus_defconfig b/configs/taurus_defconfig
index ef9e444..9a1dac7 100644
--- a/configs/taurus_defconfig
+++ b/configs/taurus_defconfig
@@ -115,6 +115,7 @@
 CONFIG_USB_GADGET_MANUFACTURER="Siemens AG"
 CONFIG_USB_GADGET_VENDOR_NUM=0x0908
 CONFIG_USB_GADGET_PRODUCT_NUM=0x02d2
+CONFIG_USB_GADGET_AT91=y
 CONFIG_USB_GADGET_DOWNLOAD=y
 CONFIG_WDT=y
 CONFIG_WDT_AT91=y